This invention relates to the removal of volatile components from a liquid stream by contact with a stripping gas, and more particularly to an improved process for removal of volatile organic components from an aqueous liquid by stripping with air or another oxygen containing gas.
Aqueous waste streams from various commercial and industrial processes are contaminated with volatile organic components. Removal of such organic components is necessary or desirable in order to meet water pollution standards. One method of removal is to strip the volatile organic component from the aqueous waste stream by contact with air, for example, in a packed stripping tower in which the air flows countercurrently to the aqueous liquid. This system is effective for removal of volatile organics such as benzene, toluene, ketones, ethers, phenol, naphthalene, acrylonitrile, oil, light petroleum fractions, and the like. So that a water pollution problem is not converted to an air emissions problem, the volatile organics are preferably burned before the effluent gas stream from the stripping tower is discharged into the atmosphere.
Since the concentration of a volatile organic contaminant in the effluent gas from the stripper is typically below flammable limits, fuel from independent sources may be burned to generate temperatures high enough for the contaminant to be consumed. These systems operate at elevated temperatures in excess of 1000.degree. C. Alternatively, and preferably in most cases, catalysts are used which cause the organic contaminants to be oxidized by combustion at low concentrations and low temperatures, for example, 300.degree.-700.degree. C., often without supply of supplemental fuels.
Preferably, the contaminated gases are preheated before being fed to either a thermal or catalytic combustion chamber. For thermal efficiency, the gases are advantageously preheated by transfer of heat from the combustion gas produced by burning the contaminants. In the case of industrial or commercial process vent streams containing organic contaminants, it is known in the art to use a regenerative heat transfer system to preheat the entering gas by recovery of combustion heat. In such a system, the contaminated gas is passed in series through a regenerative heat exchange zone, a combustion zone, and another regenerative heat exchange zone. Each heat exchange zone contains a heat storage material to which heat is transferred when the combustion gas is flowing through that zone; and flow through the system is periodically reversed. In each cycle, heat is recovered from the combustion gas by transfer to the heat storage material in the heat exchange zone downstream of the combustion zone, and the entering gas is preheated in the heat exchange zone upstream of the combustion zone by transfer of heat that had been absorbed from the combustion gas in the previous cycle. The point of cycle reversal may be dictated by attainment of a maximum acceptable temperature in the downstream heat exchange zone or a minimum acceptable temperature in the upstream zone. Destruction of volatile organic contaminants in oxygen-containing vent gases is described, for example, in Friday et al., "Selection of Treatment Process to Meet OCPSF Limitations," Environmental Progress, Vol. 10, no. 9 (August 1981), pp. 218-224. A regenerative heat transfer reaction process for destruction of organic contaminants which uses catalytic oxidation is described in U.S. Pat. No. 4,877,592.
Where ambient air is used for stripping contaminated aqueous streams that are also at ambient temperature, the equilibrium partial pressure of the organic contaminants in the gas phase may be low. If so, the concentration of organic contaminants in the effluent gas is correspondingly low, and a high gas flow rate may be necessary for effective stripping of the organic contaminant from the liquid. This translates into high capital and operating costs for the stripping tower and the blower which moves air through the tower.
Matros, Catalytic Process under Unsteady-State, (Elsevier Science Publishers, Amsterdam, Netherlands 1989) describes a regenerative heat transfer reaction system for combustion of organic components in an oxygen-containing gas stream, in which a portion of the heat of reaction is recovered in the form of high potential energy, such as high pressure steam. In this system, a portion of the reaction gas is diverted from the system and passed through an energy recovery device such as, for example, a waste heat boiler. The reaction gas leaving the boiler is returned to the reaction zone or to the regenerative heat exchange zone downstream of the reaction zone. In such a system, the generation of high potential energy is realized at the partial expense of preheating the gas entering the reaction zone. If the recoverable reaction energy substantially exceeds what is required to preheat the effluent stripping gas containing the volatile organic component to the temperature necessary to initiate the oxidation reaction, the system of the Matros article provides a salutary means for recovering the excess energy in useful form. However, the Matros article system does not deal with the problem of promoting mass transfer from the liquid to the stripping gas in the liquid stripper where the equilibrium partial pressure of the organic contaminant is low.